Anodic method for cleaning nickel and other metal surfaces for electro-plating



United States Patent Ofiice 3,362,89 Patented Jan. 9, 1968 ABSTRACT OF THE DZSOLOSURE Anodic treatment of a metal surface in an aqueous solution of concentrated lithium halide to prepare the surface for subsequent electrodeposition of a metallic coating.

The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

This invention relates to a process for cleaning metal substrates prior to electrodeposition.

Many industrial processes require electrodeposition of a metal on a substrate of the same or a different metal. Commercially pure nickel is used frequently in the chemical and process industries for the fabrication of equipment where corrosion is an important consideration Often nickel is electroplated over a less noble metal substrate of superior structural properties to provide the corrosion resistance necessary to exist in an otherwise destructive environment. A case in point is the nickel electrodeposits used to retard the oxidation of molybdenum at elevated temperatures. However, there are numerous environments in which nickel is not wholly satisfactory and an overplate of a noble metal such as platinum or palladium would improve the corrosion characteristics of the object. Previous methods for chemically cleaning nickel prior to plating platinum or palladium result in unsound deposits. These methods included: (a) a 3 to 5 minute dip in 8 parts nitric acid, 1 part hydrofluoric acid, and 16 parts water at 150 to 165 F. followed by activation for 2 minutes in concentrated hydrochloric acid, and (b) a 3 to 5 minute dip in 2 parts hydrochloric acid, 1 part ferric sulfate, and 7 parts water at 160 to 180 F. followed by activation in concentrated hydrochloric acid for 2 minutes.

Also, electrodeposition on an aluminum substrate is generally difficult. As a result, aluminum is ordinarily given a zinc immersion treatment prior to electroplating with other metals; the electrodepo-sits are thus formed on the intermediate zinc layer. However, such immersion deposits are generally less adherent than eleclrodeposits.

Electrodeposition on iron and steel are also difiicult since most conventional cleaning procedures result in voids between the substrate and the subsequent electrodeposit.

It is therefore an object of the present invention to pro vide an effective and convenient process for cleaning a metal substrate prior to electrodcposition of a metal on the substrate.

It is a further object of the invention to provide such a process for cleaning a nickel substrate prior to electrodeposition of an adherent deposit of platinum or palladium.

It is a further object of the invention to provide such a process for cleaning an aluminum substrate to allow electrodeposition of other metals thereon.

It is a still further object of the invention to provide such a process for cleaning an iron or stainless steel substrate prior to electrodeposition.

It has now been found that the above objectives may be accomplished by means of a process in which the metal substrate is given an anodic treatment in a concentrated aqueous solution of a lithium halide. Lithium chloride and lithium bromide have been found to be particularly effective in the process .of the invention.

Anodic treatment prior to electrodeposition is conventional as disclosed in Patent No. 2,457,060 to McQuire. This patent discloses the use of a dilute acid solution containing copper ion and chloride ion for anodic cleaning of nickel prior to electrodeposition of nickel on a nickel substrate. This process is, however, not effective for cleaning many metal surfaces such as the aluminum and iron or stainless steel subtrates discussed above. Furthermore, the process of McQuire is disadvantageous in requiring the presence of Cu and its periodic replenishment. Various halide solution have also been used in process for etching metals; however, such processes have not been found to provide a surface which will give an adherent electrodeposit.

The preferred solution of LiBr is prepared by mixing the salt with distilled water until a solution having a specific gravity of about 1.65 is attained. This corresponds to a concentration of 58 percent LiBr by weight which is somewhat less than its solubility at room temperature. The solution is sometimes hazy, depending upon the purity of the salt, and is moderately viscous.

The preferred LiCl solution is prepared by mixing the salt with distilled water until a specific gravity of 1.25 is attained. This corresponds to a concentration of percent by weight LiCl which is also somewhat less than the solubility at room temperature. This solution is moderately viscous.

These LiBr and LiCl solutions, employed in the specific examples to be described below, have been found to be highly satisfactory. However, the invention is not limited to these concentrations of the halide which may vary from about 30 and 20% by weight to about 65 and by Weight, respectively.

The metal substrate will generally be advantageously treated according to conventional cleaning procedures, i.e.,

degreasing, descaling, etc., prior to the anodic treatment of the invention. It is then made anodic by conventional application of suitable electrical potential as it is immersed in the halide solution. One to two minutes at an anode current density of about 1.0 to about 3.0 amperes per square inch will usually be sufficient to provide the required metal surface suitable for electroplating. On removal from the halide solution the substrate is thoroughly rinsed with distilled water and then plated.

The metal substrate may consist of cobalt, copper or brass as well as the aforementioned nickel, aluminum, iron or steel. It may be in any form such as rolled sheets, rods, etc.

A wide variety of metals may be electrodeposited according to the method of the invention. Examples are platinum metals, brass, copper, Zinc, nickel, cadmium, gold, tin or alloys of these metals.

The current density and voltage employed are not critical and will vary according to the metal being treated, degree .of cleaning desired, concentrations of solutions, etc. A range of about 0.25 ampere per square inch to about 10 amperes per square inch is generally satisfactory. The time of treatment is also not critical and will of course depend on the current density, voltage, particular substrate and degree of cleaning desired. Generally treatment is for a period of about 0.5 minute to about 5 minutes.

At a concentration of 40% by weight, the pH of the lithium chloride solution is about 5. At a concentration of 58% by weight, the pH of the lithium bromide solution is about 9. However, the pH of the solution is not critical, although an acidic solution is preferable in some instances.

The pH of such acid solutions may range up to about 5.0. The optimum value is best determined empirically. When an acidic solution is desired it is preferably obtained by addition of the corresponding halogen acid, i.e., HBr when a bromide solution is employed and HCl when a chloride solution is employed. However, other acids such as H 50 may be advantageous in certain applications.

Ambient temperature and pressure are generally satisfactory although increased temperature may be desirable to accelerate cleaning in some instances.

At the preferred current densities, when using LiBr solution a reddish-brown film of bromine forms at the anode surface and settles to the bottom. In some instances this film completely covers the surface; sometimes it develops on only certain areas of the anode surface. In either case the cleaning process is equally eflective.

As the solution is used it becomes clear and develops a brown hue due to the bromine formation. Additions of 5 to milliters per liter of fuming hydrobromic acid may be desirable to maintain the acidity of the solution. This also causes the solution to develop a brown color.

When the LiCl solution is employed a greenish-yellow film forms at the anode. This film is dissolved chlorine gas. As cleaning continues the solution takes on a green coloration. Also, a white precipitate may collect in the bottom of the solution. A small quantity (5 milliliters per liter) of concentrated hydrochloric acid may be added to maintain the acidity of the solution at a pH of about 0.9.

Though applicant does not Wish to be bound by any theory of operation, the superior results of the process of the invention are believed to result from complete removal of oxides from the substrate metal and prevention of reformation of oxides by formation of a halide of the metal forming the substrate. This halide does not interfere with the subsequent electroplating step and a surface free of oxide or other deleterious contaminants is thus provided until electroplating is commenced.

The anodic cleaning described in the examples, below, was performed in a polyethylene container using cathodes of the same material as that being cleaned. Other cathode materials and containers may, however, be employed provided they are compatible with the solutions employed and do not adversely afiect the anodic reaction. Other equipment consists of a direct current rectifier as well as ammeter and voltmeter. These devices are, of course, conventional in anodic cleaning and electroplating processes and do not constitute an essential part of the instant invention. The electroplating procedures following the anodic cleaning are also conventional and do not form a part of the invention.

The following examples are given to more specifically illustrate the invention and are not to be construed as in any way limiting the scope of the invention.

Example 1 A pure nickel specimen 6 by A by inches was cleaned and degreased with carbon tetrachloride and ethyl alcohol. An area 1 by A1 inch (0.50 square inch) was taped oil with Scotch brand pressure sensitive tape (No. 470) prior to electroplating. The specimen was immersed in the lithium bromide solution and maintained as an anode for 1.5 minutes at a current of 0.5 ampere (current density 1 ampere per square inch). It was immediately rinsed on removal, then plated to a thickness of 4 mils in a platinum plating solution. This plating solution consisted of 9.8 grams of platinum diamino dinitrite (4O grams per liter of platinum), 13.5 grams of sulfamic acid (90 grams per liter). Distilled water was added to make 150 milliliters. It was operated at a temperature of 75 C. and a current density of 13 amperes per square foot for 20 hours. The resultant deposit was hard, continuous and very adherent. Using the same procedure, adherent,

coherent deposits of palladium were also made on nickel from a palladium diamino dinitrite bath at 90 C. A similar run using the aqueous lithium chloride solution yielded similar results.

Example 2 An aluminum specimen 1 by /4 by V inch was degreased with carbon tetrachloride and ethyl alcohol. An area 2.5 by A1 inch was taped oil with Scotch brand (No. 470) electroplating tape. The specimen was immersed in dilute caustic soda at room temperature until strong gassing occurred and the aluminum surface turned black. The specimen was rinsed in distilled water and immersed as an anode in the lithium bromide solution for 1 minute at a current of 2.5 amperes (current density 2 amperes per square inch). On removal, the specimen was again rinsed in distilled water, then electroplated with copper. This aqueous bath consisted of 150 grams per liter of copper sulfate (CtISO -SH O), 27 milliliters per liter of concentrated sulfuric acid, and 50 milliliters per liter of ethyl alcohol. It was operated at room temperature and at a current density of 10 amperes per square foot until a l mil deposit had been attained. After plating, the specimen, in the area of the electroplate, was repeatedly bent through an angle of l. With minor exceptions, the

crack which developed continued through the electrodeposit into the substrate. The deposit was very adherent even after this repeated flexing.

Example 3 A stainless steel specimen 4 by 1 by 4 inches was degreased with ethyl alcohol, dried, and a 2 by 1 inch taped of with Scotch brand electroplating tape No. 470. The specimen was anodically treated in the aqueous lithium bromide solution for 1 minute at a current of 4 amperes (current density 1 ampere per square inch). A brownishred film of bromine covered the anode. Being denser than the solution, the bromine settled to the bottom of the container. The specimen was rinsed thoroughly in distilled Water, then plated with brass from a copper-zinc cyanide bath at 45 C. This aqueous plating solution consisted of 52.5 grams per liter of cuprous cyanide, 30 grams per liter of zinc cyanide, grams per liter of sodium cyanide, 45 grams per liter of potassium sodium tartrate (Rochelle salt), and 30 grams per liter of sodium carbonate. The pH was adjusted to lG.5 with either sodium hydroxide or ammonium sulfate. The bath was operated at 5 to 30 amperes per square foot at a temperature of 45 C. until a deposit 1 mil thick was attained. This deposit was very adherent and metallographic examination revealed a continuous deposit-tosu.bstrate interface. Using the same procedure, adherent and coherent deposits of brass were also made on Armco iron sheet.

What is claimed is:

1. A method for obtaining improved adherence of a metallic electrodcposit to a metallic substrate comprising anodically treating the substrate prior to electrodeposition, in a bath consisting of a concentrated aqueous solution of a lithium halide, said halide being present in an amount of about 2G to about 65 percent of the weight of said solution; and subsequently electroplating a metallic coating on said anodically treated metallic substrate.

2. Method of claim 1 in which the lithium halide is lithium bromide.

3. Method of claim 2 in which the concentration of the lithium bromide is from about 30 percent to about 65 percent by weight.

4. Method of claim 3 in which the concentration of the lithium bromide is about 58 percent by weight,

5. Method of claim 1 in which the lithium halide is lithium chloride.

6. Method of claim 5 in which the concentration of the lithium chloride is from about 20 percent to about 45 percent by weight.

7. Method of claim 6 in which the concentration of the lithium chloride is about 40 percent by Weight.

8. Method of claim 1 in which the substrate is nickel.

9. Method of claim 8 in which the electrodeposited metal is from me group consisting of platinum palindium.

10. Method of claim 1 in which the substrate is aluminurn.

11. Method of claim 10 in which the electrodeposited metal is copper.

12. Method of claim 1 in which the substrate is from the group consisting of iron and stainless steel.

13. Method of claim 12 in which the electrocleposited metal is brass.

14. The method of claim 1 in which the anode current density is from about 0.25 ampere per square inch to about 10 amperes per square inch, and in which the treatment is carried on for about 0.5 minute to about 5 minutes.

References Cited UNITED STATES PATENTS 2,457,061 12/1948 McQuire 20432 2,473,163 6/1949 McCoy 204-42 2,593,043 5/1952 Eichner 1486.27 2,619,454 11/1952 Zapponi 204-43 3,082,136 3/1963 Finn 15617 3,096,261 7/1963 Mekjean 204141 HOWARD WILLIAMS, Primary Examiner.

IGHN H. MACK, Examiner.

W. VAN SISE, Assistant Examiner. 

